1. Field of the Invention
This invention is directed to a hand-held laser range finder, and, more particularly, to such a range finder with a TV-type viewing field and a reticle representing the laser footprint and with a target quality bar graph display.
2. Description of the Related Art
Laser range finders have been manufactured for a number of years. Typically such range finders send a laser pulse and receive the pulse as reflected from a target. An internal clock monitors the time difference between the transmitted and received pulses, halves the time difference and multiplies it by the speed of light to thereby derive a distance from the range finder to the target.
One problem with prior art range finders is that the measured time periods are so short that extremely fast clocks and very sophisticated circuitry are required for accurate distance ranging. Ideally a range finder should be capable of accuracy within approximately 1 meter at a 1000 meter range. For each additional meter of range, a laser light pulse takes approximately 3 nanoseconds extra in transit. This, then, is the shortest time period which must be measured. Thus, for distances of .+-.1 meter to be accurately measured, a clock with a frequency of 160 MHz is required. Processors capable of operating at this clock speed are expensive, which effectively prices typical prior art range finders well beyond the mass consumer market.
A method has been developed to obviate the need for such high frequency clocks in a laser range finder. This method, developed by Laser Tech, Inc. of Colorado, is described in application for U.S. patent Ser. No. 08/375,945, entitled LASER RANGE FINDER HAVING SELECTABLE TARGET ACQUISITION CHARACTERISTICS AND RANGE MEASURING PRECISION, filed Jan. 19, 1995, which is incorporated herein by reference. In the Laser Tech circuit, a series of 30 or more individual laser pulses are emitted each time a trigger switch is engaged. A charging circuit is triggered with the emission of each pulse whereby a capacitor is charged at a first, relatively rapid rate during the time of flight for each pulse. After each return pulse is detected, a discharging circuit is triggered which discharges the capacitor at a second, much slower rate. A microcontroller times the discharge period and calculates a range based upon the time of discharge by the use of a calibrated formula. In one example, the capacitor discharge rate was 1000 times slower than the charging rate, thus allowing an 8 MHz crystal oscillator to be used as a timer clock source to yield an accuracy of .+-.1 meter at a 1000 meter range. An automatic noise threshold circuit sets a minimum threshold noise level which allows reliable detection of reflected laser pulses and a dithering circuit selectively provides increased accuracy.
In the Laser Tech circuit a minimum number of valid reflected pulses must be received from the series of emitted pulses in order to yield a desired ranging accuracy. The number of received pulses is dependent upon the "target quality", i.e. the reflective capabilities of the target. This varies greatly among target materials, and other factors such as atmospheric conditions, open field versus brushy surroundings, etc. also effect the number and quality of received pulses. However, without information relating to target quality, a user of such a range finder would not be able to isolate the reason for a lack of a distance reading, i.e. low battery, equipment malfunction, target quality, etc.
It is clear then, that a need exists for a laser range finder which utilizes relatively slow speed clocks to accurately sense range distances to within .+-.1 meter. Such a laser range finder should include an indication of target quality such that a user of the range finder can be apprised of the reflective quality of the target, thus enabling him or her to select a different target feature in the event of a low quality target indication.